1. Field of the Invention
The present invention relates to a capacitor microphone unit that can have improved sensitivity while maintaining its excellent directional frequency response characteristics up to a high tone range and a capacitor microphone using such a capacitor microphone unit.
2. Description of the Related Art
A capacitor microphone unit is an electro-acoustic converter including a diaphragm and a fixed electrode facing each other with a certain space provided therebetween and utilizing a mechanism in which the capacity of a capacitor formed of the diaphragm and the fixed electrode changes when the diaphragm vibrates upon receiving sound wave. FIGS. 10A, 10B and 11 exemplary illustrate a conventional capacitor microphone unit.
As illustrated in FIGS. 10A, 10B, and 11, a cylindrical casing 101 incorporates a diaphragm ring 102, a diaphragm 103, a spacer 104, a fixed electrode 105, an acoustic resistor 106, a terminal 107, an insulating substrate 108, and a ring screw 109 in this order. The diaphragm 103 is made of a thin resin film having a surface on which a piece of metal is vapor deposited. The peripheral portion of the diaphragm 103 is fixedly adhered to the diaphragm ring 102. The casing 101 has a flange directed inward on one end, i.e., the front end that is on the left side in FIG. 11. The diaphragm ring 102 is in contact with the peripheral portion of the flanged portion. The spacer 104 of a thin ring-shaped plate is interposed between the diaphragm 103 and the fixed electrode 105, thereby forming a gap corresponding to the thickness of the spacer 104 between the diaphragm 103 and the fixed electrode 105. With this structure, an electret capacitor microphone can be provided by forming an electret layer on either of the surfaces of the diaphragm 103 and the fixed electrode 105 that are facing each other.
The terminal 107 penetrates the center hole of the insulating substrate 108 to have its rear end protrude towards the rear side of the microphone unit while the head portion on the front side of the terminal 107 is in contact with the fixed electrode 105. The acoustic resistor 106 is held by the insulating substrate 108 and defines an acoustic resistance in a space reaching the rear surface of the diaphragm 103 through a hole in the fixed electrode 105 from an acoustic terminal formed of a space provided in the insulating substrate 108. The ring screw 109 is screwed into the inner periphery at the rear end of the casing 101 to press the insulating substrate 108 towards the front side of the casing 101. With the above described elements being pressed with this pressing force, the diaphragm ring 102 is in contact with the inward-directed flange of the casing 101 and the elements are held in the casing 101 in a mutually pressed state.
The diaphragm ring 102 is electrically connected to the diaphragm 103 and the casing 101. Thus, a sound signal as a result of electro-acoustic conversion can be output from the casing 101 and through the terminal 107 electrically connected to the fixed electrode 105. Generally, an impedance converter such as a field electric transistor (FET) is provided to lower the impedance of the sound signal that is weak but has high impedance. An output circuit of a capacitor microphone using the above described capacitor microphone unit is exemplary illustrated in FIG. 12. In the output circuit, the fixed electrode 105 is connected to the input terminal of an impedance converter 110, the diaphragm plate 103 is connected to the ground side, and the primary coil of a transformer 111 is connected between an output terminal of the impedance converter 110 and the ground. The ends of the secondary coil of the transformer 111, respectively serving as a hot-side and a cold-side output terminals for a balanced output, are each connected to a microphone cable via a connector. The ground side is connected to a shielding cable of the microphone cable. Thus, balanced sound signal can be output.
Directional frequency response characteristics of a conventional capacitor microphone unit having the above described structure are depicted in FIG. 13. The thickest characteristic line represents the directional frequency response characteristic measured at the front of the microphone unit, i.e., the position that is not offset from the central axis of the microphone unit. The second thickest characteristic line represents the directional frequency response characteristic measured at a side of the microphone unit, i.e., the position offset from the central axis of the microphone unit by 90 degrees. The thin characteristic line represents the directional frequency response characteristic measured at the rear of the microphone unit, i.e., the position offset from the central axis of the microphone unit by 180 degrees. The characteristics were measured in accordance with EIAJ standard. Capacitor microphones are demanded to have improved sensitivity without degrading directional frequency response characteristics especially in a high frequency domain.
It is desirable that sensitivity of a microphone is high. Higher sensitivity can be provided to a capacitor microphone with the following possible measures:    1. increasing the driving force;    2. lowering the impedance of the microphone unit; and    3. providing the microphone unit with a diaphragm plate having a larger area.
It is most practical to provide higher sensitivity by providing the microphone unit with a diaphragm plate having a larger area among the measures. Unfortunately, this degrades the directional frequency response characteristics in a high frequency domain, i.e., sensitivity in a high frequency domain is degraded. Therefore, the inventors of the present invention have proposed an invention disclosed in Japanese Patent Application Publication 2006-5710 that relates to a capacitor microphone with which intrinsic noise can be reduced without degrading directional frequency response characteristics in a frequency domain including a high frequency domain. In the capacitor microphone according to such an invention, a plurality of small-diameter unidirectional capacitor microphone capsules (microphone units) is apposed, connected in parallel, and is connected to a single impedance converter.
The capacitor microphone described in Japanese Patent Application Publication 2006-5710 can solve the problem only to a certain level. More specifically, sensitivity over an expected level cannot be obtained because multiple microphone capsules are connected in parallel.
Therefore, the assignee filed a patent application, Japanese Patent Application Publication 2009-151768, on a capacitor microphone in which multiple microphone units are connected in series in an arrangement in which diaphragms of the respective microphone units are arranged to be on the same plane and an output from an impedance converter connected to one of the microphone units drives the ground side of another microphone unit. This application (hereinafter, referred to as prior invention) is not yet published at the point of the application of the present invention.
The capacitor microphone according to the prior application can improve the directional frequency response characteristics up to a high frequency domain while improving the sensitivity.
On the other hand, with the capacitor microphone according to the prior application, a new technical problem to be solved arises. Specifically, the size of the microphone applying this configuration is large because multiple microphone units are physically arranged in series.